nginx installation fails

[schwemmer]

When following the instructions to build Sipi from source I run into an issue when trying to install nginx. While the error message is cryptic, the solution is simply to stop Apache before trying to install nginx (cf. here: https://askubuntu.com/questions/764222/nginx-installation-error-in-ubuntu-16-04).

Maybe this (or something similar) could be added as a note to the instructions:

Note: if you have Apache running on your system you need to stop it before installing nginx or the installation will fail